This invention relates to an electrical switch, and in particular to an electrical switch used in the start circuit of a dynamoelectric machine.
There are applications for dynamoelectric machines, such as motors and generators, which require operation in environments containing high levels of airborne contaminants including particulate and abrasive matter. In industrial applications there often are a myriad of airborne industrial pollutants and contaminants present in areas of motor operation. Moreover, even in the home environment, an application of a motor such as in a power saw, exposes the motor to wood dust created during a sawing operation. This, too, can effect the operation of the motor powering the saw. Even insects are of concern in some applications.
Contaminants are of particular concern because of their possible effect on the switching operation that occurs during motor starting operation. Contaminants, regardless of their source, have direct access to the switch contacts used in the motor start circuits. The effect of contaminants can vary from simple arcing across switch contacts to switch failure. In applications where a high level of pollution or contamination exists, the entire motor or generator is sealed to prevent such failure. When a machine is sealed, however, heat generated during its operation is not as easily dissipated as it is from an unsealed machine.
To address the heat dissipation problem, sealed machines are constructed larger in size than comparable open machines and are thus, heavier, and costlier. The size, weight and cost penalties associated with sealed machines is good reason to use open machines where applications permit, even though the operating environment will subject the machine to environmental contamination and performance degradation. It will be understood that where an open motor is used in an environment known to be contaminated with particulate matter such as saw dust, or chemical compounds which are, for example, acidic in nature, a price will be paid at some point. Materials will deteriorate and degrade, and performance will, in time, suffer. With respect to the present invention, unprotected switching functions quickly degrade in an atmosphere laden with sawdust or other industrial dusts, or other particulate matter that deposits on the switch contacts and interferes with switch action. Switching may become intermittent and unreliable, and arcing will cause pitting on the contacts and other detrimental effects.
The types of motors normally employed in the above described environments are frequently split phase or capacitor start induction motors. They generally employ a centrifugal actuator acting upon a switch of the type contemplated by the present invention to control application of power to the motor's start circuit. When power is first applied to such a motor, the centrifugal actuator, which is mounted on the motor shaft, is mechanically in contact with one side of the switch, causing it, in turn, to close upon the other side of the switch and connect the motor start winding to the motor power source. With the start winding thus connected, the rotor begins to turn, gradually increasing its speed. Upon reaching a predetermined operating speed, the centrifugal actuator pulls away from the side of the start winding switch that it is contacting. The switch opens and removes the start winding from the power source. The main motor winding provides for motor operation thereafter. If the start switch and its associated actuator are exposed to a contaminated operating environment, their performance is so effected that they may be incapable of functioning. The motor may fail because the start winding cannot get power, or the start winding cannot be disengaged and either fails or causes nuisance trips of any associated motor protection device.
Many types of switches, and ways for protecting them are known to those familiar with the art. One example is disclosed in U.S. Pat. No. 4,414,443 ('443), the disclosure of which is intended to be incorporated herein by reference. While the prior art in general and the '443 patent in particular work well for their intended purposes, their use is not without drawbacks. In particular, the structure used to protect the switch is relatively expensive, It also is difficult to maintain alignment of the switch components with prior art designs, particularly over the useful life of the motor.
The present invention offers a simple, easily producible, and inexpensive means for protecting a set of motor switch contacts used in a highly contaminated environment. The invention remains remarkably free from the adverse effects of the operating environments above described. In the present invention, one switch contact located intermediate the ends of a flexible conductive arm, is surrounded by a protective metal shell or cap. The cap is open in the direction of a mating contact which is on the end of a post that is attached to a terminal board on which the entire switch assembly is mounted. A soft, pliable boot surrounds the post and fits about the cap, enclosing the mating contact on the flexible arm. Thus the entire switching environment; that is, the environment to which the switch contacts are exposed, is sealed off by the mating cap and boot. It has also been found that the combination cap, post, and boot help maintain alignment of the switch contacts during repetitive opening and closing of the switch. This combination of elements has resulted in extremely long switch life, even in adverse operating environmental conditions.
It is an object of the present invention to provide an improved switch for dynamoelectric machines operating in contaminated environments.
Another object of the invention is to provide a switch having contacts enclosed so as to inhibit the effects of the external environmental contaminants.
Still another object of the invention is to provide a protected switch for use in dynamoelectric machines operating in contaminated environments which is simple in design and operation, and inexpensive to produce.
Yet another object of the invention is to provide a motor starting switch assembly for use in a dynamoelectric machine employing a centrifugal actuator.
A further object of the invention is to provide a switch assembly which maintains switch contact alignment so to increase the useful life of the assembly.
These and other objects of the present invention will be more clearly understood after reference to the drawings and the description accompanying them which follows.